1. Field of the Invention
This invention relates to a method for manufacturing semiconductor devices of polycide gate structure, and more particularly to a method used for manufacturing semiconductor devices having a thin gate oxide film.
2. Description of the Related Art
In a MOS transistor manufacturing process, semiconductor impurity is ion-implanted into a semiconductor substrate to form source and drain regions. At this time, charges caused by the ion-implantation are injected into the gate electrode so as to charge the gate electrode (gate charge). The gate charging increases the potential of the gate electrode so that the gate oxide film may be subject to dielectric breakdown by the electric field created between the substrate and the gate electrode when the gate oxide film is thin. In order to solve this problem, the ion-implantation is generally effected with the gate electrode layer and the silicon substrate held in contact with each other. With the gate electrode layer kept in contact with the silicon substrate, charges caused by the ion-implantation will not be stored in the gate electrode but discharged into the substrate.
In order to set the gate electrode layer in contact with the silicon substrate, it is necessary to remove the contact portion of the gate electrode layer by the photolithographic process, for example, and form a contact hole for connection with the gate oxide film. In the process of forming the contact hole, a photoresist film of a predetermined pattern is formed on the thin gate oxide film and the gate oxide film is selectively etched to form the contact hole using the resist film as a mask. However, formation of the photoresist layer on the thin gate oxide film may significantly degrade the quality of the surface of the gate oxide film and increase the weak-point density. Further, material mixed into the photoresist layer may be injected into the gate oxide film to lower the electrical stability of the gate oxide film.
As described above, in the conventional manufacturing method, there are problems that the gate oxide film may be damaged in the manufacturing process and unwanted material will be injected into the gate oxide film.